An active matrix liquid crystal display includes a TFT substrate (TFT array substrate) provided with thin-film transistors (TFTs) as switching devices, a transparent conducting film and a wiring unit including gate, source and drain lines; a counter substrate disposed opposite to and separated by a space from the TFT substrate and provided with common electrodes; and a liquid crystal layer filling up the space between the TFT substrate and the counter substrate. The transparent conducting film is, for example, an indium tin oxide film (ITO film) of indium oxide (In2O3) containing about 10% by mass tin oxide (SnO) or an indium zinc oxide film (IZO film) of indium oxide containing 10% by mass zinc oxide.
FIG. 1 is a schematic, enlarged sectional view of a liquid crystal panel included in such a liquid crystal display. This liquid crystal panel includes a TFT array substrate 1, a counter substrate 2 disposed opposite to the TFT array substrate 1, and a liquid crystal layer 3 sandwiched between the TFT array substrate 1 and the counter substrate 2 and serving as a light modulator layer. The TFT array substrate 1 includes an insulating glass substrate 1a, thin-film transistors (TFTs) 4 arranged on the glass substrate 1a, transparent conducting film 5 forming pixel electrodes, and a wiring unit 6 including scanning lines and signal lines.
The counter substrate 2 includes common electrodes 7 formed on a surface thereof facing the TFT array substrate 1, a color filter 8 facing the transparent conducting film 5 (pixel electrodes), and a shading film 9 facing the thin-film transistors (TFTs) 4 and the wiring unit 6 of the TFT array substrate 1.
Polarizers 10 are attached to the outer surfaces of the TFT array substrate 1 and the counter substrate 2, respectively. The counter substrate 2 is provided with a liquid crystal alignment film 11 for aligning liquid crystals with a predetermined direction.
An alignment direction with which the liquid crystal molecules are to be aligned is controlled by an electric field created between the counter electrode 2 and the transparent conducting film (pixel electrodes) 5 to modulate light passing through the liquid crystal layer 3 formed between the TFT array substrate and the counter electrode 2. Thus, the quantity of light that passes through the counter substrate 2 is controlled to form an image.
The TFT array is driven through a TAB tape 12 connected to the TFT array by a drive circuit 13 and a control circuit 14.
Shown also in FIG. 1 are a spacer 15, a sealing material 16, a protective film 17, a diffusion plate 18, a prism sheet 19, a light guide plate 20, a reflecting plate 21, a back light 22, a holding frame 23 and a printed wiring board 24.
FIG. 2 is an enlarged view of a part A shown in FIG. 1 and including the wiring unit 6 electrically connected to the transparent conducting film (pixel electrodes) 5. The gate line 26 shown in FIG. 2 is a layered wiring film formed by superposing a film of a metal, such as Mo or Cr, or an aluminum alloy film of, for example, An Al—Nd alloy, and a film of a refractory metal, such as Mo, Cr, Ti or W. A source line 28 and a drain line 29 (referred to as source•drain lines in some cases) are layered film each of a single-layer film of pure aluminum (Al) and a film of the foregoing refractory metal as mentioned in, for example, Patent documents 1, 2 and 3.
The film of the refractory metal is used for the following reasons. When the transparent film (ITO film) 5 is connected directly to the film of pure aluminum or an aluminum alloy, such as an Al—Nd alloy, forming the gate lines and the source•drain lines, aluminum is oxidized and a aluminum oxide layer having high resistivity is formed between the transparent conducting film and the pure aluminum film or the aluminum alloy film, such as an Al—Nd film. Consequently, contact resistance between the signal line and the transparent conducting film increases to deteriorate the image quality of displayed images.
Since aluminum is an element that can be readily oxidized, an aluminum oxide film is formed in the atmosphere. An aluminum oxide film having high resistivity is formed particularly by oxygen used for forming the transparent conducting film of a metal oxide or by oxygen produced in a film forming process.
The oxidation of the surface of a pure aluminum film or an aluminum alloy film is prevented by using a barrier metal (refractory metal) layer to ensure satisfactory contact between a pure aluminum or aluminum alloy film and a transparent conducting film.
An additional process for forming the metal barrier layer is necessary to build a structure including the metal barrier layer, and an additional apparatus for forming the metal barrier layer is necessary in addition to a sputtering apparatus for forming a film for forming gate lines and source•drain lines. Increase in the cost and the reduction of productivity resulting from the formation of the metal barrier layer have become problems with the progress of the reduction of the cost of the liquid crystal display panel by mass production. There has been a demand for electrode materials and manufacturing processes effective in omitting the metal barrier layer in recent years. The inventors of the present invention have previously proposed an aluminum alloy wiring film capable of simplifying the metal barrier layer forming process and of being in direct contact with the transparent conducting film in Patent document 4.
Disclosed in Patent document 5 is a technique that enables the omission of a metal barrier layer by subjecting a drain electrode to a surface treatment process, such as a plasma process or an ion implantation process. Disclosed in Patent document 6 is a technique that enables the omission of a metal barrier layer by using a layered wiring film formed by superposing a second layer containing impurities, such as N, O, Si and C, to a first layer for forming gate, source and drain electrodes.
The length of wiring lines has increased with the progressive increase in size of liquid crystal display panels and the resistivity of signal lines has increased greatly accordingly. The increased resistivity causes RC delay which is can deteriorate the image quality of displayed images. Accordingly, demand for wiring lines having low resistivity has increased and wiring materials having low resistivity, such as pure Al, are used for forming wiring lines having low resistivity.
When a transparent conducting film is brought into direct contact with pure aluminum or aluminum alloy wiring lines, contact resistance increases and the display quality of the screen is deteriorated. Such a problem arises because aluminum is oxidized easily as mentioned above, the surfaces of wiring lines are readily oxidized in the atmosphere, and an insulating aluminum oxide film is formed on the surfaces of the wiring lines by the oxidation of aluminum by oxygen used for forming the transparent conducting film of a metal oxide. If an insulating film is formed between the signal lines and the transparent conducting film, contact resistance between the signal lines and the transparent conducting film increases and hence the display quality of the screen deteriorates.
A metal barrier layer used in conventional liquid crystal display panels prevents the oxidation of the pure aluminum wiring lines or aluminum alloy wiring lines and ensures the satisfactory contact between the transparent conducting film and the pure aluminum wiring lines or the aluminum alloy wiring lines. Since refractory metal, such as Mo, Cr, Ti or W, used for forming the metal barrier layer has high resistivity, increase in the resistance of wiring lines due to the use of the metal barrier layer is unignorable.
Difficulty in fine processing is one of disadvantages of using a metal barrier layer. Usually, it is considered to be desirable that the sectional shape of a wiring line is tapered at a taper angle in the range of about 45° to about 60° considering satisfactory coverage. Since a wiring line of a layered film including a metal barrier layer have layers of different metals. Therefore, the metal layers of different metals are etched at different etch rate, respectively, and in some cases, has a sectional shape as schematically shown in FIG. 3. Since metals respectively having different potentials are joined, the galvanic corrosion of the wiring line occurs and, in some cases, the wiring line has a sectional shape shown in FIG. 4. When wiring lines have such a sectional shape, a passivation film cannot be formed in satisfactory coverage and the operating characteristic of the TFTs (thin-film transistors) is deteriorated.
Moreover, for example, the heat resistance of pure aluminum wiring lines is unsatisfactory and protruding defects called hillocks are formed. When the hillocks penetrates the insulating film, wiring lines are short-circuited. Thus the reduction of the yield of a production line due to the use of pure aluminum is a practically significant problem.    Patent document 1: JP H4-20930 A    Patent document 2: JP H6-12503 A    Patent document 3: JP 2001-350159 A    Patent document 4: JP 2004-214606 A    Patent document 5: JP H11-238934 A    Patent document 6: JP H11-284195 A